1. Field of the Invention
The present invention relates to a composition for forming a hard coating layer.
2. Discussion of Related Art
Recently, thin display devices using flat display devices such as liquid crystal display devices, organic light emitting diode display devices or the like have received much attention. Especially, these thin display devices are implemented in the form of a touch screen panel and are widely used in a variety of smart devices characterized by portability such as smart phones, tablets, PCs and a diversity of wearable devices.
These portable display devices based on a touch screen panel includes a window cover for protecting displays on a display panel to protect a display panel from scratches or external shocks. In most cases, tempered glass for display is used as the window cover. Tempered glass for display is thinner than general glass, but has high strength and scratch resistance.
However, a heavy weight of tempered glass is unsuitable for reducing the weight of portable devices. Further, tempered glass is difficult to implement unbreakable characteristics due to its vulnerability to external shocks. Tempered glass may only bend to a limited degree, and thus is unsuitable as a material for flexible displays which are bendable and foldable.
In recent, various researches on an optical plastic cover ensuring flexibility and impact resistance, and having strength or scratch resistance equal to those of tempered glass have been conducted. Generally, examples of the optical plastic covers which are more flexible than tempered glass include polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polyacrylate (PAR), polycarbonate (PC), polyimide (PI), etc. However, these polymer plastic substrates exhibit insufficient physical properties of hardness and scratch resistance as compared to tempered glass used as a window cover for protecting displays, and also has insufficient impact resistance. Accordingly, many attempts to complement the physical properties by coating these plastic substrates with a composite resin composition have been in progress.
In a general hard coating process, a composition including a resin having photocurable functional groups such as a (meth)acrylate group, an epoxy group or the like and a curing agent or a curing catalyst and a reactive additive is used. Especially, a composite resin having a high functional group may be used as a window cover for protecting displays having improved hardness and scratch resistance when an optical plastic base material film is coated with the composite resin.
However, in the case of general photocurable composite resins having a high functional group of a (meth)acrylate or epoxy group, it is difficult to implement high hardness equal to that of tempered glass, and a curling phenomenon due to shrinkage occurs upon curing the resin. Flexibility is also insufficient, and thus these resins are unsuitable as a window substrate for protecting displays to be applied to flexible displays.
A plastic substrate is disclosed in Korean Laid-open Patent Application No. 2013-74167.